Porous media are useful in many separation and adsorption technologies, such as chromatography. One particular type of porous media, porous membranes, are used for a variety of applications. Porous membranes have a first porous surface, a second porous surface, and a continuous porous structure that extends throughout the membrane from the first to the second surface. The continuous porous structure includes the bulk material matrix and the network of pores. The interface separating the bulk matrix from the pore volume (i.e., the surface of the interior pore network) is known as the interstitial surface.
Porous membranes can be classified as “microporous” membranes or “ultrafiltration” membranes on the basis of the size of the pores of the membrane. Generally, the range of pore sizes for microporous membranes is considered to be from approximately 0.05 micron to approximately 10.0 microns, whereas the range of pore sizes for ultrafiltration membranes is considered to be from approximately 0.002 micron to about 0.05 micron. These pore sizes refer to pore diameter for circular or approximately circular pores, or to a characteristic dimension for non-circular pores.
The pore size of a membrane can be denominated by the size of the smallest species (particle or molecule) that cannot pass through the membrane above a specified fractional passage. A common rating is below 10% passage, which corresponds to a 90% cutoff or retention. Other methods are known to those skilled in the art, including image analysis of scanning electron microscopy to obtain pore size distribution characteristics. Microporous membranes are typically used to remove particulates from liquids and gases. An important application of microporous membranes is in sterile filtration of pharmaceutical solutions to remove any bacteria that may be present in the solution. Microporous membranes are also used as sterile gas vents, which allow gas flow but prevent airborne bacteria from passing through the filter. Ultrafiltration membranes are generally used in applications where retention of smaller species is desired. For example, ultrafiltration membranes are used in the biotechnology industry to concentrate proteins, and in diafiltration applications to remove salts and low molecular weight species from protein solutions. Both ultrafiltration and microporous membranes can be fabricated in several forms, including sheets, tubes, and hollow fibers.
Porous membranes are made from a variety of materials, polymers being the most common. Many commercial membranes are made from engineering plastics, such as polyethersulfone, polysulfone, polyvinylidene fluoride, polyethylene, polytetrafluoroethylene, polypropylene and so forth, to take advantage of their robust thermal, mechanical, and chemical-resistance properties. Unfortunately, these materials are hydrophobic and have a high propensity to adsorb biomolecules.
In general, a hydrophilic membrane which is readily wet with an aqueous solution is preferred for filtration of aqueous solutions. In contrast, with hydrophobic membranes, contact with a low surface tension prewetting liquid followed by water exchange is required to start permeation. This not only introduces added material cost to the process, but any wetting liquid must be exhaustively flushed, which greatly increases the possibility of contamination, process time and cost. A hydrophobic membrane which is not wet with an aqueous solution can be utilized to filter organic solutions or gases.
In addition to permeability and retentive properties, porous membranes have other requirements that are dictated by the nature of the application. For example, they must have sufficient mechanical strength to withstand operating pressures and temperatures. In applications where cleanliness is a major requirement, as in the pharmaceutical or microelectronics wafer-making industry, the amount of material that extracts from the membrane in use must be very small. In applications where the membrane comes in contact with biomolecules, the membrane surface must be resistant to biomolecule adsorption. A biomolecule resistant surface is a surface that adsorbs or binds minimal amounts of biomolecules, such as proteins and nucleic acids; specifically, a surface that adsorbs less than about 30 micrograms of IgG per square centimeter of membrane area as measured by the IgG binding test described herein. It is greatly preferred that a membrane surface be maximally biomolecule resistant, to reduce permeation loss from fouling by surface adsorption or pore blockage, and to prevent product loss by irreversible adsorption or associated biomolecule denaturization.
In many applications, porous membrane come in contact with highly alkaline solutions in cleaning or sanitation operations. Thus, the membrane must have sufficient chemical resistance to withstand such contact without loss of filtration properties, desirable surface properties or mechanical properties.
Porous media and membranes are used with functional surfaces. Functional surfaces have chemical groups or moieties which react with, or adsorb or absorb specific species in the fluid contacting the media or membrane. Examples of such groups are positively or negatively charged groups, affinity ligands such as antibodies or antigens, metal affinity ligands, and hydrophobic interaction groups.
To impart the aforementioned properties to porous media or membranes, manufacturers typically modify the membrane surface (i.e., the first and second surfaces and the interstitial surface) of the bulk matrix material making up the porous media or membrane to make the surface hydrophilic and biomolecule resistant or hydrophobic. This is done by a variety of procedures that coat, attach to, or otherwise cover the surface of the bulk matrix material with a hydrophilic polymer or hydrophilic group or with a hydrophobic polymer or hydrophobic group. While such modification can solve some problems related to the hydrophobic or high biomolecule binding of the bulk matrix material surface, it also can add other problems. For example, such modifications increase the amount of material able to be extracted from the membrane during use, and the modification material can have low tolerance to exposure to alkaline solutions. In addition, in many applications membranes are heated, either by wet heat as in autoclaving or steam sanitization, or by dry heat, as in a drying step. It has been observed that such heating will reduce hydrophilicity and decrease biomolecule resistance of some modified surfaces to the extent that they cannot be used for their intended purpose.
Some membranes of the prior art are made by modifying the surface of preformed porous membranes with cross-linked hydroxyacrylates, where the crosslinking monomer is a difunctional acrylate (“Case A membranes”). Such membranes have excellent resistance to biomolecule adsorption, excellent heat stability of the biomolecule resistance, and acceptable flow loss (i.e., the reduction in flow or permeability compared to the unmodified membrane). However, while these membranes have acceptable cleanability characteristics (i.e., the ability to clean the membrane by washing such that residual extractable matter (“extractables”) is lowered to an acceptable level, it was found that to lower extractables to below a certain level, about 2 microgram per square centimeter using the TOC test (described herein in the “Methods” section), a very extensive extraction regime was needed. In addition, these membranes were sensitive to strong alkaline solutions in that they lost their initial permeation characteristics and low extractables characteristics.
Much of the prior art describes the use of hydroxyl containing monomers, usually carbonyl ester containing acrylate polymers, to produce membrane surface modifications having hydrophilic character and high resistance to protein binding. However, it is known that polymers from such monomers are not resistant to strong alkaline solutions. For example, a solution of 1.0 normal sodium hydroxide will hydrolyze the carbonyl containing acrylate polymers to acrylic acid containing polymers. Such acrylic acid containing polymers are ionically charged under certain pH conditions, and will attract and bind oppositely charged proteins or biomolecules, thus increasing sorption and membrane fouling. In addition, acrylic acid containing polymers swell in water to an extent that they constrict pore passages, thus reducing membrane permeability and productivity. Moreover, polymers from hydroxyl containing monomers, such as hydroxy acrylates, further react in strong alkaline solutions and degrade into soluble low molecular weight fragments, which dissolve away and expose the underlying substrate porous media or membrane.
Various methods of modifying porous membranes are known in the art. For example, U.S. Pat. No. 4,618,533 discloses and claims a composite porous thermoplastic membrane which comprises a porous membrane substrate having an average pore size between about 0.001 and 10 microns formed of a first polymer, the substrate being directly coated on its entire surface with a cross-linked second polymer formed from a monomer polymerized in situ with a free radical initiator on the substrate, where the composite porous membrane has essentially the same porous configuration as the membrane substrate.
U.S. Pat. No. 4,944,879 discloses a composite porous membrane having desired bulk properties on which is coated a cross-linked polymer having desired surface priorities. The cross-linked surface polymer is produced from a crosslinkable monomer or polymer by energy from an electron beam in the absence of a chemical polymerization initiator.
Similar modified porous media are disclosed in U.S. Pat. Nos. 4,906,374, 4,968,533, and 5,019,260, in which hydroxyl containing polymeric material is derived from monomers having hydroxyl groups and moieties characterized by having one polymenzable unit of unsaturation, such as hydroxy or hydroxy-forming substituted acrylates or methacrylate esters. Polymers from such monomers are known to lack resistance to degradation by strong alkaline solutions.
Chapman et al (J. Am. Chem. Soc. 2000, 122, 8303–8304) describe the use of self assembled monolayers (SAM) to screen functional groups for protein resistance. They report several functional groups to be protein resistant, including N-substituted amide functionalities.
U.S. Pat. Nos. 4,695,592 and 4,678,813 describe a process and product for a hydrophilized porous polyolefin membrane with a crosslinked polymer, which is composed of 50% or more of diacetone acrylamide monomer.
Iwata et al (J. Membrane Sci. 1991 55 119–130) report acrylamide coatings of membranes that have temperature responsive properties (TRP), specifically polyacrylamides, and particularly poly(N-isopropylacrylamide (polylPAA)). Iwata report the graft polymerization of homopolymers of polylPAA and copolymers with acrylamide to a first surface of a PVDF membrane. However, they do not cross-link the polymers, as that would impede the polymer TRP.
U.S. Pat. No. 5,929,214 to Peters et al, describes porous monoliths functionalized and/or grafted with TRP polymers, including non-crosslinked copolymers of polylPAA. These membranes are designed to adsorb biomolecules, and the Peters et al. patent does not teach the production of protein or biomolecule resistant surfaces.
U.S. Pat. Nos. 5,217,802 and 5,286,382 describe methods for rendering the surface of polymeric membranes hydrophobic.
It can be seen that practitioners attempting to develop optimized membranes for sterile filtration and other applications in the pharmaceutical and biotechnology industries must overcome significant problems. Facing stringent cost, performance and safety requirements, a practitioner must use materials and develop manufacturing methods that produce membranes with not only optimized flow and retention characteristics, but be economical to produce, meet cleanliness criteria, be stable to the various chemical environments which are commonly encountered, and be very resistant to biomolecule adsorption. Thus, it would be desirable to have a membrane modification that results in a hydrophilic, biomolecule resistant surface that is heat stable, which is resistant to degradation by alkaline solutions, and which has very low levels of material capable of being extracted therefrom. It would also be desirable to provide membranes having highly hydrophobic surfaces for use, for example, in filtering gases.